Ecosystems

An ecosystem consists of a community of
organisms and the non-living factors that influence them. The living and
non-living elements interact to support and maintain a balance between
plant and animal communities within the ecosystem.

It is possible to think of the whole earth as
an ecosystem. The biosphere
includes all the earth's landmasses, oceans and atmosphere, and all the
animals, plants, insects, birds and micro-organisms living in them. But because
the biosphere is so large and all-encompassing and its relationships so
complex, we normally study smaller ecosystems.

We can group together ecosystems adapted to
similar climate conditions into global ecological regions called biomes,
each of which supports its own typical range of plant and animal species.

The
map above shows the major biomes of the world, while the pyramid below shows
the relationship between biomes and climate.

As well as the vast global regions known as biomes
there are two other categories of ecosystem:

Micro ecosystems, also known as microclimates, are
subdivisions of meso systems. They inhabit very small areas such as a water
droplet, a single leaf, or the cool, damp space under a log - where the
conditions may be quite different from the habitat as a whole but just right
for the organisms that live there.

Ecosystems are shaped by two types of
environmental factors: non-living (abiotic) and living (biotic) factors.

Abiotic
factors are the elements of an
ecosystem that are non-living. Nevertheless, they still have an affect on
the ecosystem. Water, temperature, relief, soil type,
fire, and nutrients
are all examples of abiotic factors.

Biotic
factors are the living elements
of an ecosystem, i.e. plants and animals. All biotic factors require energy
to survive. These living organisms form a community
within an ecosystem.

The community within an ecosystem is linked together by food
chains. Biotic factors become linked in a food chain when they eat one
another. The start or bottom of a food chain is made up of producers,
such as plants and algae. Producers are at the start, or the bottom, of the
food chain because they do not eat other biotic factors to get the energy they
need to survive. Instead of taking energy from food, producers get energy by
converting it from carbon dioxide and water using sunlight. This process is
called photosynthesis.

Other biotic factors in the food chain survive by eating the producers.
This gives them the energy that the producers obtained through photosynthesis.
These biotic factors are called consumers because they eat other organisms to
get their energy, rather than taking it direct from sunlight. Biotic factors
that eat consumers are also consumers and they are often called carnivores.
There are four types of consumer:

Omnivores are organisms that eat both plant (producer) and animal (consumer)
matter to gain energy.

Decomposers are organisms that feed on the remains of dead plant and animal
matter. They help to speed up the process of decay. They also assist in
recycling nutrients back to producers in nutrient cycles.

Energy moves around within an ecosystem, from factor to factor, as
producers and consumers are eaten. The flow of energy in this way is what we
call the food chain. This movement of energy through a food chain can be shown
using a flow diagram, as shown in the following diagram.

The producers (algae) obtain energy through
photosynthesis from the sun. Energy then passes up the food chain as the
algae are eaten by a herbivore (mosquito larvae) and carnivores (dragonfly
larvae and perch).

In real life, food chains are not this simple
as several different, but interrelated (connected) food chains combine together
into a food web. The food chains are interrelated because
animals do not eat the same food every meal. The following diagram shows how
food chains form complex food webs.

In the woodland food web the energy
stored in the grass (producer) passes to the fox and the owl, through the mouse
food chain or the rabbit food chain. The energy from the acorn passes to the
fox and the owl through a second mouse food chain.

********************************************************************

Example of an ecosystem

The following diagram shows an oak tree
ecosystem. Note that different living things inhabit different zones or layers
from the bottom to the top:

First there is the roots, soil and leaf-litter zone beneath the
tree. Here decomposers such as bacteria, woodlice, and earthworms feed off
last year's leaves and acorns, and fungi grow on its roots

Next is the trunk layer, which provides shelter or food to insects,
caterpillars and larvae.

Finally comes the branches, leaves and canopy. In this zone bees
gather pollen and nectar, fungi grow on the leaves, gall wasps and moths
lay their eggs, and squirrels gather acorns. Small birds such as bluetits
hunt the moth larvae; and sparrowhawks hunt the small birds.

An oak tree ecosystem. Each zone of the tree
is home to a distinctive community of organisms.

Each of the organisms in this ecosystem has a
particular way of fitting into the oak tree environment - they each occupy a niche
within the ecosystem. For example the blue tits and the squirrels, though they
both inhabit the same tree, do not directly compete for food: the squirrels
feed on acorns, while the tits feed on moth larvae. The two species occupy
different niches within the oak ecosystem.

********************************************************************

Adaptation

The competition between organisms means that
those who are best adapted are most likely to survive and reproduce. Natural
selection means that over many generations organisms become progressively
better adapted to their environment.

Adaptation in extreme habitats: three examples:

Camels are
well adapted for survival in the desert. They have:

the ability to store a lot of water, and to lose very little via
urination and sweating

the ability to tolerate body temperatures up to 42 degrees C

a large surface area / volume ratio - maximising heat loss

a hump which can store scarce food as fat without insulating the
body

thick fur on the top of the body to provide shade, and thin fur on
the rest of the body to allow easy heat loss, and

large, flat feet well-suited for walking on sand

Polar bears are
well adapted for survival in the arctic. They have:

a thick layer of fur for insulation against the cold

a thick layer of blubber for further insulation, and also as a food
store

a small surface area / volume ratio, to minimise heat loss

a greasy coat, which sheds water after swimming

large feet to spread the load on snow and ice, and

the ability to move fast on both land and water, while pursuing
their prey

Cacti are
well adapted for survival in the desert. They have:

spines instead of leaves, which minimize surface area and therefore
evaporation, and also...

protect them from animals which might eat them

stems which can store water, and

widespread root systems, which can collect water from a large area

********************************************************************

Competition and
co-operation

Competition

Habitats have finite amounts of the resources
needed by living organisms, such as food, water and space, and all organisms
strive to reproduce themselves and increase their numbers. Sooner or later the
demand for these resources is going to exceed supply, and organisms have to
compete with each other to get them.

Plants typically compete with each other for:

light (for photosynthesis)

water, and

nutrients (minerals)

Animals typically compete with each other for:

food

water

mates (so they can reproduce), and

living space

It is
this competition between organisms that enables natural selection to take
place, by favouring chance mutations conferring a slight advantage in the race
for limited resources.

Co-operation

Some organisms find that they are better able
to survive and reproduce by living closely together with another organism of a
different species. This type of interdependence between organisms is known as symbiosis.

Barnacles which attach to a whale or scallop shell. The
barnacles get a home and transport, and the whale or scallop is not unduly
affected. This type of symbiosis, where one organism benefits and the other
suffers no harm, is called commensalism.

Lichens are formed by algae and fungi living together. Algae
can photosynthesise and make food which is shared by the fungus. The fungus in
turn shelters the algae from a harsh climate. This kind of mutually beneficial
co-operative relationship is called mutualism.

A
tapeworm lives inside another animal, attaching itself to the
host's gut and absorbing its host's food. The host loses nutrition, and may
develop weight loss, diarrhoea and vomiting. This kind of one-sided symbiosis
is called parasitism. Usually parasites do not kill the host
before they move on, as this would cut off their food supply.

********************************************************************

The threat to biodiversity

The world is an astonishingly diverse place,
inhabited by millions of different plant and animal species, with probably
millions more that have not yet been discovered.

The term biodiversity refers
not only to the sheer number of different species, but to all the genetic
variations within and between species - and all the differences between the
many, many habitats and ecosystems that make up the earth's biosphere.

Why is it important to maintain the huge
variety of life? Some answers are that :

·we
have a moral responsibility to look after the planet and its resources, rather
than simply use them up

biodiversity provides us with many direct benefits - e.g. clean air
and water, food, medicines, fertile soil, and pollination for our crops

biodiversity represents the earth's total gene pool - a source of
future variation which is vital for all species

reduction in biodiversity in ecosystems may reduce climatic
stability, for example by upsetting the balance between intake and output
of carbon dioxide

Impact of humans

By far the biggest threat to the variety of
life is posed by human exploitation of the environment. Our impact on the
global environment is greater than that of any other species because of:

Our technologies: we use tools and techniques which can change
the shape of the earth in a short space of time (e.g. clearing forests or changing
the course of rivers)

Our population is increasing at a phenomenal rate. There are 6
billion of us now and by 2050 we are expected to number around 9 billion. Our
sheer numbers will mean that even small activities multiplied 9 billion times
will have a huge impact on the environment

Consumption and waste: we consume vast amounts of natural
resources (e.g. water, fossil fuels) and produce vast amounts of waste (e.g. greenhouse
gases). Both pose a threat to other forms of life - as well as to
ourselves!